1. Field of the Invention
This invention generally relates to spread spectrum communications systems and, more particularly, to a reliable two-way communications link between mobile units and fixed base facilities.
2. Description of the Prior Art
Spread spectrum communications offer several advantages in communications applications requiring high reliability. These include low density power spectra and interference rejection. In the case of interference rejection, the interference may be accidental, that is, simply part of the environment of the communication system. In a specific application, the communications environment may include many potential reflectors giving rise to severe multipath interference. Such multipath interference typically insinuates deep nulls in the form of frequency selective fading. Spread spectrum communications is an ideal countermeasure to this difficulty.
There are several types of spread spectrum systems including direct sequence digital systems, frequency hopping systems, time hopping systems, pulsed frequency modulated (or chirp) systems, and various hybrids. Of these, the direct sequence digital systems and frequency hopping systems are perhaps the more widely implemented. In a direct sequence digital system, a pseudorandom code generator is used to modulate a frequency modulated carrier. In a frequency hopping system, a coherent local oscillator is made to jump from one frequency to another.
Spread spectrum communications are often asymmetric in cost and complexity. For example, suitable spread spectrum signals may be generated with relatively low complexity circuitry. The challenge is then how to accomplish demodulation of such signals. Usually it is necessary to construct a demodulator which can generate or otherwise process a wideband signal in order to recover the narrowband message. Synchronization is often the most difficult and costly aspect of a spread spectrum communications system.
In order to simplify the receiver in a spread spectrum communications system, it is known to transmit a reference that may be used in place of synchronization and tracking at the receiver; that is, the coded reference is generated and transmitted with the intended information signal. Since the local reference is generated at the transmitter, the receiver need not have a code sequence or other local reference generator.
The invention relates to the direct sequence digital type of spread spectrum communications system using a transmitted reference. A description of this and other types of spread spectrum communications systems may be found, for example, in Spread Spectrum Systems, 3rd Ed., by Robert C. Dixon, John Wiley & Sons (1994), and Spread Spectrum Communications, Vol. 11, by M. K. Simon et al., Computer Science Press (1985).
In some applications, the communication channel may have severe multipath due to the presence of many reflectors. While spread spectrum systems are tolerant of multipath and provide interference proiection, such systems typically require a complex receiver structure. One reason for the complexity is the need to synchronize a local reference that is used to "despread" the signal. Also, this synchroniztion process makes it more difficult to provide "burst-mode" communications because the transmitter must wait for the receiver to synchronize its local reference before sending information.
SUMMARY OF THE INVENTION
An object of the invention is to provide a simple and inexpensive method and apparatus for implementing a spread spectrum communication system that provides both multipath and interference protection.
The invention eliminates need for a local reference by transmitting a reference that the receiver can use to perform despreading. The method and apparatus of the invention is in the class of systems known as "transmitted reference" spread spectrum systems. In general, a transmitted reference system sends both a message signal and a reference signal to the receiver. The message signal contains the information to be communicated, which has been spread through multiplication with a wide band "spreading waveform". The reference signal consists of the spreading waveform itself, which the receiver can use to despread the message signal and recover the information.